The physiological function and mode of regulation of transglutaminases are being studied as to their role in the formation of provisional stroma during tissue or bone fracture repair, in the cross-linking of extracellular matrix proteins during cell-cell interaction, and in the modulation of specific cellular processes. The coagulant layer formed at injury sites is one of the vital elements of hemostasis and diathesis. The major constitutent of this coagulant gel is fibrin. A number of other proteins that are known to be involved in the cell attachment reaction are crosslinked to fibrin, e.g., fibronectin, thrombospondin, vitronectin, osteonectin, etc. The crosslinking reaction is modulated at inflammatory or wound sites by oxidants, reducing agents and plasma proteins, all of which effect the catalytic activity of factor XIIIa. Cellular transglutaminases in terminally differentiated epidermis catalyze the crosslinking of numerous proteins in keratinocytes to form cornified envelope. A large majority of cytosol transglutaminase was shown to occur as an inactive znymogen in the granular layers of epidermis and appears to be activated during the apoptotic process in terminally differentiated epidermis. In the stratum distendum of oral epithelium, a large majority of the transglutaminase activity which is associated with the cell membrane readily dissociates and catalyzes the formation of a cornified cell envelope and the crosslinking of salivary proteins with oral epithelium to form mucosal pellicle.